Various methods for removing sulfur dioxide from a combustion exhaust gas have been heretofore proposed, including a dry method using a variety of adsorbents and a wet method using aqueous alkali sulfite solution. In the wet method, an exhaust gas is introduced into an aqueous alkali sulfite solution for reacting sulfur dioxide contained in the aqueous exhaust gas with the alkali sulfite to obtain the alkali bisulfite solution, to which calcium carbonate is added for forming calcium sulfite, thus sulfur dioxide being removed in the form of calcium sulfite. The reaction mechanism in the method can be expressed by the following reactions (1) and (2) where sodium sulfite is used as an alkali sulfite: EQU SO.sub.2 + Na.sub.2 SO.sub.3 + H.sub.2 O = 2NaHSO.sub.3 1. EQU 2NaHSO.sub.3 + CaCO.sub.3 = CaSO.sub.3.sup.. 1/2H.sub.2 O + Na.sub.2 SO.sub.3 + CO.sub.2 + 1/2H.sub.2 O 2.
however, the wet method employing an aqueous alkali sulfite solution in removing sulfur dioxide from the exhaust gas has many disadvantages in that a large amount of calcium carbonate is required for conducting the above reaction (2). Calcium carbonate is hardly soluble in water so that, when it is added to the aqueous alkali bisulfite solution for the reaction expressed by the formula (2), the solid-liquid reaction takes place with only extremely low reactivity, with the result that unreacted calcium carbonate remains in the reaction system in a large amount. Moreover, since the produced calcium sulfite is also almost insoluble in water, the calcium sulfite is undesirably mixed with the above-mentioned unreacted calcium carbonate, making it difficult to recover calcium sulfite in a highly pure form.